1. Field of the Invention
The present invention relates to systems and methods for providing calibration and accuracy checking for a breath tester. Specifically, utilizing small dispersed drops of alcohol or a water and alcohol mix to simulate exhausted human breath to a breath tester.
2. Description of the Related Art
For the purposes of public safety on the roads and elsewhere, there is a need to make sure that individuals are not operating potentially dangerous machines (such as automobiles) while they are impaired by the effects of alcohol consumption. To try and prevent people from driving drunk, most states have enacted laws which impose fines or other criminal penalties if individuals have a breath or blood alcohol level above a certain amount. In order to effectively enforce these laws, it is necessary to be able to measure the alcohol concentration in human breath and compare the results against legal limits. There are a variety of measuring instruments used for determining the concentration of alcohol in human breath ranging from small hand held devices to larger bench top units and machines built into cars or certain machinery. Since a determination of breath alcohol above the legal threshold can result in criminal penalties, loss of a job, or other sanctions, the accuracy of these instruments is paramount.
Great care and effort is taken by owners and managers of evidential breath testing equipment to ensure proper calibration as well as follow-up accuracy checks at generally regular intervals. In attempts to eliminate the labor time of this testing and concerns about human error in the testing, manufacturers of breath testing equipment often offer automated or semi-automated methods for doing calibrations and accuracy checks. Some users of breath alcohol test equipment, such as Motor Vehicle Law Enforcement, may even require an automatic accuracy check every time they test a human subject and sometimes even before and after the human subject test simply to make sure that the device is reading correctly and will supply court-admissible evidence.
There are generally different standards used when calibrating breath testers. As breath (containing alcohol or not) is a vapor comprising exhalation gases and vaporized substances, instruments that measure alcohol concentration in this breath vapor generally need standards to be provided in a similar form for accurate calibration. Calibration gases of many sorts are well known in many applications including breath testing. In breath testing, the calibration standards are generally of two types, wet and dry. Wet standards include water vapor; dry standards do not. Some argue that wet standards are better because they include moisture like human breath and are therefore more representative. However, commercial providers of both wet and dry standards generally advertise +/−2% accuracy of calculations with actual breath.
In either case, the alcohol concentration of measurement interest is in a carrier gas such as air, breath, or nitrogen. A typical breath ethanol concentration which would result in illegal driving in most states is 200 parts per million (ppm) or more. That is 200 parts ethanol per million parts of carrier gas regardless of the carrier gas composition. Therefore, the standards generally provide samples which contain very close to 200 ppm to make sure the dividing line is correctly calibrated.
Wet standards have a long history in breath testing, are well accepted, and the liquids used in them can be certified by chemical analysis against NIST traceable standards. The standards are prepared by combining known amounts of ethanol and water in a partially filled jar that is accurately heated to 34° C. These heated jars are sold commercially and are referred to as Simulators. At equilibrium, the quiescent headspace above the jar contains a vapor with a known concentration of ethanol along with nearly 100% relative humidity at that temperature.
By introducing sober human breath or air from another suitable source into the jar (by blowing or injecting gas into the liquid) the known concentration of ethanol vapor exits the headspace and can be introduced into a breath tester at which point a measurement may be taken.
Dry standards, by contrast, have no water vapor included with them. This is because dry standards are prepared with carrier gases such as nitrogen or argon and are supplied in pressurized tanks ranging from 500-2500 psi. At these pressures, if water vapor were included in amounts similar to human breath concentrations in practical field use, the water would condense out of the gas, trap ethanol, and cause wholly inaccurate results. The dry gas standards are typically certified by measurement against NIST-prepared standards.
In automated wet testing, the above-mentioned Simulators generally have input and output ports. Typically, a Simulator will sit alongside a breath test machine, normally on a desktop. The output of the Simulator is plumbed into the instrument such that when gas is pumped into the Simulator input (either from a tester blowing into it, or from an associated gas tank or pump), a vapor of known ethanol concentration will be presented for measurement or calibration in the same manner human breath would be. Typically, an electric pump is used to pump ambient air into the Simulator for this purpose. The pump may be internal to the breath tester, part of the Simulator itself, or an entirely separate component. Typically, gas is pumped through a Simulator for 4-10 seconds in order for a measurement to be completed. This pump time varies depending on the flow rate and the amount of instrument volume that has to be purged of ambient gas before a measurement is taken to ensure the measurement is taken of the carrier gas with the correct concentration of ethanol.
Every time a sample is taken from a Simulator, some of the ethanol in the liquid replenishes lost ethanol from the headspace. However, over time, the equilibrium concentration of ethanol provided by the Simulator decreases from its originally intended value as ethanol is slowly lost to the ambient air due to the carrier gas (and the carried ethanol) being exhausted from the breath tester. Some breath test instruments use recirculation systems that take the ethanol vapor provided by the Simulator output, after it exits the breath tester's measurement chamber, or manifold, and pumps it back into the Simulator inlet, instead of using ambient air to provide the simulated exhalation. This greatly reduces any effects of lost ethanol from the Simulator causing lower concentrations to be provided over time since used ethanol is not exhausted to the ambient, but is returned to the Simulator.
Whether using recirculation systems or not, care must be taken to avoid any condensation of water from the Simulator output until the concentration of ethanol is measured by the breath tester. Otherwise, the alcohol in the gas will be less than intended due to ethanol being condensed from the gas. To avoid condensation, various elements or tubes in the instrument are generally heated prior to measurement.
It must be noted that the using Simulators for portable instruments or in on-site calibration tests can be problematic. They are subject to splashing, tipping over, and operate properly within a very limited ambient temperature range due to their complicated design which is necessary for accuracy. Further, they are not really designed for easy or efficient transport, and that tends to limit their use to controlled settings.
The dry gas standards are provided in a variety of types of high-pressure cylinders. A typical size of a tank is approximately 1 liter or more. These cylinders are typically equipped with pressure regulators where the high tank pressure is regulated down to a much lower delivery pressure to the breath tester to better simulate the pressure provided by human breath. Often, an electronic shut-off valve will allow delivery of the low-pressure calibration gas to the measurement chamber on demand.
Compared to wet standards, the dry standards offer some advantages. Dry gas delivery systems generally represent a less complex system hardware design to provide automated calibrations and accuracy checks than the wet standards. The dry gas system is generally easier for instrument owners to manage and maintain and the dry gas system is certainly more amenable to a portable system. Specifically, since the only major components of a dry gas system are the tank and regulator, they are pretty easily portable and are not as affected by movement or situation as wet systems. The dry gas tanks will eventually run empty, but no recirculation system is required to keep the value stable throughout the tank's lifetime.
However, dry gas standards have several factors that complicate their use. First of all, they require a compensation for barometric pressure in the breath tester. The concentration of dry gas standards follow the ideal gas law, and the measured value will change with barometric pressure changes due to elevation or weather. Also, if a dry gas system has a leak, it is possible to lose a significant amount of gas before a problem is noticed. Furthermore, some users (especially mobile ones) have concerns about the safety of transporting even relatively small high-pressure gas tanks which, even while filled with generally nonflammable gas, are potentially explosive due to their high pressure. Lastly, as stated earlier, the dry gas contains no water vapor. Some who are skilled in the art believe that a water component to the calibration gas is essential, because water vapor is a large constituent of human breath and it would therefore be possible to challenge the reading of a breath tester which has only been calibrated using a dry gas system.